As is known, helicopters are normally equipped with a number of transmissions for transmitting power from one or more turbines to the main and/or tail rotor, and/or from the turbine to accessory devices, i.e. for supplying the energy needed, for example, to operate on-board equipment.
One transmission is normally interposed between the turbine and the main rotor drive shaft.
The final reduction stage of this transmission is normally an epicyclic reduction gear for transmitting power with adequate torque and speed to the rotor shaft.
An epicyclic reduction gear substantially comprises:                a first or sun gear, which rotates about a fixed first axis;        a fixed second or ring gear coaxial with the first axis; and        a number of planet gears, which mesh with the sun and ring gears, and rotate about respective movable second axes parallel to the first axis.        
The above epicyclic reduction gear also comprises a planet carrier, which rotates about the first axis and is connected to the planet gears.
In this way, in addition to rotating about their respective second axes, the planet gears also revolve about the first axis of the sun gear.
More specifically, the sun gear is connected to an input shaft, and the planet carrier acts as an output shaft connected to the rotor shaft.
In other words, mechanical power enters the epicyclic reduction gear via the sun gear, and is transmitted at the right torque and speed to the rotor shaft by the planet carrier.
The sun gear, ring gear, and planet gears are all mounted on an annular bottom supporting plate, and are oil lubricated by nozzles fitted to two bars fixed to the plate, radially with respect to the first axis, and extending from the outer periphery of the sun gear.
Because of the location of the nozzles, in two predetermined positions with respect to the sun gear, and the size of the planet gears, the space for oil flow from the nozzles is extremely limited, thus preventing constant lubrication of the meshing sun gear and plane gear teeth, and so resulting in poor lubrication, particularly of the sun gear, which normally performs a number of meshing cycles per revolution.
US 2011/0299974A1 discloses an epicyclic gear train.